Skip main thread test in ThreadSafeProgressMonitor

update(int) is only invoked from a worker thread, in JGit's case
this is DeltaTask. The Javadoc of TSPM suggests update should only
ever be used by a worker thread.

Skip the main thread check, saving some cycles on each run of the
progress monitor.

Change-Id: I6cb9382d71b4cb3f8e8981c7ac382da25304dfcb

org.eclipse.jgit.test/tst/org/eclipse/jgit/lib/ThreadSafeProgressMonitorTest.java

@@ -104,9 +104,11 @@ public class ThreadSafeProgressMonitorTest {
 		assertEquals(42, mock.value);
 
 		pm.update(1);
+		pm.pollForUpdates();
 		assertEquals(43, mock.value);
 
 		pm.update(2);
+		pm.pollForUpdates();
 		assertEquals(45, mock.value);
 
 		pm.endTask();

org.eclipse.jgit/src/org/eclipse/jgit/lib/ThreadSafeProgressMonitor.java

@@ -157,10 +157,7 @@ public class ThreadSafeProgressMonitor implements ProgressMonitor {
 	}
 
 	public void update(int completed) {
-		int old = pendingUpdates.getAndAdd(completed);
-		if (isMainThread())
-			doUpdates();
-		else if (old == 0)
+		if (0 == pendingUpdates.getAndAdd(completed))
 			process.release();
 	}